1. Field of Invention
This invention relates to phased array antennas which have delay lines between the transmit/receive cells and the input for the radar signal to be transmitted.
2. Description of the Prior Art
Phased array antennas are comprised of a plurality of transmit/receive cells typically arranged on a series of parallel rows in an array. When the antenna is in a transmit mode the radar signal must be distributed over the cells. Usually all cells do not receive the signal at the same time. The art has developed binary fiber optic delay lines, known as BIFODELs, which carry radar signals to and from the transmit/receive cells. These BIFODELs have been designed and selected so that the time delays between signal arrivals at selected cells are known. Typically, one BIFODEL will serve a group or set of transmit/receive cells called a transmit/receive module.
Future high-performance phased array antennas will be required to have large scan angles, wide instantaneous bandwidths (100s of MHz), center frequencies anywhere from the UHF to the X bands, and multiple beam capability. The actual number of transmit/receive modules depends on the system mission as well as its operating frequency, and typically is in the 10.sup.2 -10.sup.4 range for all airborne, ground, and shipboard radars. Similar requirements exist for multi-function, front-end systems, which are expected to have even larger bandwidths because of the integration of radar, ECM and COM.
To satisfy the wide bandwidth requirements of such phased array antennas true time delay frequency-independent steering techniques must be used. Optical fiber is an excellent medium for both the delay generation and signal distribution because: (i) it can store large bandwidth analog signals (.about.100 GHz) for long hours (10s of .mu.s), (ii) it has low attenuation (&lt;0.1 db/km) which is flat over radio frequencies up to 100 GHz, (iii) it allows the remote processing of phased array antenna signals, (iv) it has excellent transmission stability by virtue of the small ratio of signal bandwidth to optical carrier frequency, (v) it allows optical wavelength multiplexing (.lambda.-MUX) to minimize the number of lines in the phased array antenna feed link, (iv) it is a non-conducting dielectric and so does not disturb the RF field, is secure, and EMI immune, and (vii) it is flexible, it has low mass, and small volume.
It can be shown that the straightforward implementation of true time delay for large phased array antennas results in very large amounts of hardware that reduces the overall practicality of the true time delay concept. Specifically, the hardware complexity is proportional to the product of the number of antenna elements (K) and the number of different steering angles (R). In practice K and R are in the 10.sup.2 to 10.sup.4 and 10.sup.2 to 10.sup.3 ranges, respectively. Thus, innovative techniques are required for compressing the hardware complexity with respect to both K and R.
The most efficient hardware compression with respect to R is accomplished via the use of binary fiberoptic delay lines. In a BIFODEL the optical signal is optionally routed through N fiber segments whose lengths increase successively by a power of 2. The various segments are addressed using a set of N 2.times.2 optical switches. Since each switch allows the signal to either connect or bypass a fiber segment, a delay T may be inserted which can take any value, in increments of .DELTA.T, up to the maximum value, T.sub.max, given by: EQU T.sub.max =(2.sup.0 +2.sup.1 +. . . 2.sup.n-1) .DELTA.T=(2.sup.N -1) .DELTA.T (1)
Note that the BIFODEL may be implemented with a combination of fiber and/or free space delays, and offers log.sub.2 level compressive fiber/switch complexities (M.sub.f/s): EQU M.sub.f/s =log.sub.2 R. (2)
Unfortunately, the BIFODEL concept alone does not solve the overall hardware complexity problem since a K-element phased array antenna requires K different BIFODELs.